Feasibility study of crystallization process for water softening in a pellet reactor

One of the undesirable characteristics of some groundwater sources is hardness, which has some adverse effects on water pipes, boilers and soap consumption. Therefore several treatment processes have been introduced to remove or reduce the hardness from hard waters. One of the new innovations in this regard is crystallization process. Hardness can be removed from hard waters by growth of calcium carbonate crystals in a fluidized bed reactor called pellet reactor. The design, setting up, starts up and reaching optimal condition for calcium carbonate crystallization process in a pellet reactor which has been initially seeded with sand as a crystal citation can be a successful solution for treatment of hard waters. The chemistry of pellet softening process is essentially the same as conventional softening process, instead of precipitation of calcium carbonate which does not have any useful consumption and must be removed once in a while some how, crystallization process in a fluidized bed reactor produces pure solid grain of calcite. These pellets can be used in some industries and are in fact an economic commodity. For this purpose a pilot was set up as a crystallization reactor and water with total hardness ranging from 150 up to 500 mg/l as CaCO₃ was fed to the system with a flow rate of 24 l/hr. To remove hardness, caustic and soda, lime milk was fed to the system separately at first stage at then together. The formation of CaCO₃ crystals on the surface of sands represented the reduction of hardness. An analysis is made for the characteristics of a softed water resulted from the use of caustic soda, lime milk or both as regent.

Keywords
Hardness, pellet reactor, softening, calcium carbonate, crystallization